1. Field of the Invention
The present invention relates to a die of a die compacting assembly for compacting a powdered material which is used in the field of powder metallurgy, and more particularly to a die which is suitably used in manufacture of a powder compact of high density, compacting of a hard powder such as magnetic alloy powder and the like, or compacting of a soft powder easily adhering to the die such as aluminum powder.
2. Description of the Related Art
A die compacting assembly for compacting a powdered material comprises a die unit including a die cavity corresponding to the contour of a pellet or powder compact to be produced, and upper and lower punches adapted to be applied to top and bottom of the die cavity. It can also comprises a core rod for providing a hole on the powder compact, as necessity arises. The die compacting assembly is assembled in a press machine and used as powder compacting apparatus. The powdered material is filled into the die cavity which is defined by the die cavity and the lower punch, and the upper punch is relatively moved to the lower punch to press the powdered material, whereby the powdered material is compressed between the upper and lower punches to form a compact of the powdered material. The powder compact is then ejected to the powder-charged side of the die cavity and stripped from the die unit.
Density of the powder compact is determined by the compacting pressure, and a powder compact having high density is often required in manufacture of machine components having high strength. However, it necessitates due consideration to the strength of the die unit corresponding to the high compacting pressure. Moreover, in a case where a hard powder such as a magnetic alloy powder prepared by the quench and solidification method is to be compacted in high density, the powder compacting is more difficult. Additionally, as the powder compacting is repeated, the inside wall of the die cavity is gradually worn at the area to which the pressure is concentrated, and the die cavity is swollen in the middle. As a result, the ejection resistance of the formed compact is increased, and the obtained compact is often flawed or cracked. Moreover, in the case of compacting a powdered material likely to be adhered to the wall of the die cavity such as aluminum powder, it is extremely difficult to manufacture a compact of high density, in beautiful appearance and with small variation.
In the above-described circumstances, the applicant of the present application has previously suggested a powder compacting die and a compacting method using the die which are disclosed in Japanese Patent Application Laid-Open No. 5-320705, in which a compact of the powdered material compressed in high density is favorably ejected from the die by lower pressure. The applicant has also suggested a method of compacting a sintered product of high density at high efficiency, which is disclosed in Japanese Patent Application Laid-Open No. 11-140505. The powder compacting die of the former publication is characterized by taper of 1/5,000 to 1/1,000 which is provided on the die cavity for expanding the width of the die cavity at the side which the formed compact is removed, and the compacting method using the die is characterized by using upper and lower punches which are fitted in the die cavity to press the powdered material, and particularly using the punch of the larger bore side in the majority of the compressing process or at least in the final stage of compression. The powder compacting method of the latter publication is characterized by combining a die lubrication method in which a film of a lubricant is formed on the inside wall of the die cavity, and a powder lubrication method in which the amount of the powder lubricant to be added is suppressed to 0.3% or less, preferably to 0.05 to 0.3%.
When the powdered material is compressed, the powdered material is compressed by the pressure applied by punching. However, at the same time, friction occurs on the inside wall of the die cavity between the inside wall and the powdered material being pressed, so that the compressing force effecting on the powdered material from the punching pressure is reduced by that friction. Moreover, the force necessary for removing the powder compact from the die cavity after the die compacting is proportional to the friction between the wall of the die cavity and the compact. Therefore, if the friction between the compact and the wall of the die cavity is decreased, it becomes possible to eject the compact with a smaller pressure and obtain a compact of higher density at high yield. In this connection, it has been conventionally known that treatment of the surface of the die with CVD or the like is effective as means for reducing the friction at the die cavity, and this method has been widely employed.
On the other hand, for the die structure for the powder compacting, generally, a shrink fit die which is formed by shrink fitting a ring (die holder) of structural alloy steel to the outer circumference of the die body having a die cavity is widely used for the purpose of reinforcement of the die body. However, if the surface of the shrink fit die is subjected to surface coating treatment, the shrink fitting force is lowered. To the contrary, if the ring is shrink fitted after the die body is subjected to surface coating treatment, the dimensions of the die cavity determined on the coating is changed due to the effects of heat for shrink fitting and the dimensional precision is therefore impaired.
With these problems in mind, it is therefore an object of the present invention to provide a powder compacting die in which the wear resistance is enhanced, the friction of the inside wall of the die cavity is reduced, a compact of powder compressed in high density is ejected with a lower pressure than that in the conventional art, and sintered products of high density are possibly produced efficiently.
In order to achieve the above-mentioned object, a die for compacting a powdered material, according to an aspect of the present invention comprises: a die body having a penetrating die cavity to which a couple of punches are adapted to apply for pressing the powdered material to be supplied to form a compact, the die cavity being provided with a coating composed of a material which is selected from the group consisting of titanium carbide, titanium nitride, alumina, titanium cyanide, hafnium nitride, chromium nitride, tungsten carbide and DLC; and a die holder having a bore into which the die body is shrink fitted, the die holder being composed of a steel material which is prepared by tempering at a temperature higher than a treatment temperature at which the coating is provided on the die cavity.